Several investigations have pointed to a possible need for distinguishing between "brightness" and "sensitivity" as they are defined, respectively, by reference to brightness-matching (estimation) vs. increment-threshold procedures. This project is designed to provide further evidence relating to whether brightness and sensitivity are mediated by common mechanisms through a comparison of the spatial Broca-Sulzer effect and Sensitization effect at brief exposure durations. Each of these effects denotes a trend in the intensity-area relationship (i.e., negative coefficients of spatial summation) that is not predictable from previous threshold-level studies. Specifically, the former effect refers to the finding that, for stimuli which are fixated foveally and viewed at suprathreshold levels, a disc-shaped stimulus subtending 2'--3' appears brighter than a larger stimulus of the same retinal illuminance. The sensitization effect refers to the fact that increasing the diameter of an adapting-field increases rather than decreases the increment-threshold sensitivity of the visual system. Westheimer previously reported an attenuation of the phototopic sensitization effect with an adapting-field duration of 10 msec. In contrast, the present research has indicated little, if any, attenuation of the spatial Broca-Sulzer effect at durations as brief as 10 msec. This suggests the possibility of different mechanisms underlying the two effects. If the spatial Broca-Sulzer effect is interpreted to reflect the operation of an inhibitory mechanism acting selectively to depress the brightness of the larger-diameter stimuli, then it seems likely that the type of inhibition involved is similar to that thought responsible for producing the Mach band phenomenon.